In electrical systems, there is typically a power source that produces power, and an electrical load that consumes the produced power. The various sources and loads often have different electrical properties and require power electronics to transfer power between components. A power converter is an electrical device for converting electrical energy from one form to another, such as between alternating current (AC) and direct current (DC), between different voltages or frequencies, or a combination of these. A specific type of converter, called an inverter, changes DC to AC. In a drive system, a voltage source inverter (VSI) may be used to transfer real power from a DC power source to an AC load.
In a three-phase AC system, the common-mode (CM) voltage can be defined as a voltage difference between the power source ground and the neutral point of a three-phase load. If the load is an AC motor, the neutral point of the load means the stator neutral of the motor. Inverters generate CM voltages relative to the power source ground that cause coupling currents through parasitic capacitances inside the motor (load). The main source of bearing currents is the capacitance-coupling currents that return via the motor bearings back to the ground. The bearing damage in inverter-driven motors is mainly caused by the shaft voltage and the bearing currents created by the common-mode voltage.
Conventional VSIs consist of two or three phase-legs, each of which is a series connection of two semiconductor switches. During normal operation, the output potential of each phase leg with respect to ground is continuously switched between a positive and a negative DC bus potential based on a pulse width modulation of the switches. As a result, the CM voltage imposed on the load also exhibits six step-changes during each switching cycle. Each step-change induces a charging or discharging current into the parasitic capacitances between load and ground that leads to issues such as electromagnetic interference emissions, inverter losses, and motor damage. CM chokes and filters may be used to mitigate the CM voltage induced issues. However, since CM chokes have to block a significant amount of CM voltage, they are bulky in size, and constitute a large portion of the product cost.